Hen pecked to death by flock

[TheOddOneOut]

The run is about 12x12

That’s way, way to small.If they really did peck her to death… You’ll have another victim soon if that is not corrected.

 

[TheOddOneOut]

12 x 12 is really small for that many birds, particularly with roo's and multiple species.
Without a necropsy it's impossible to know why the bird actually died, it could have been something hidden and not obvious, they are really good at hiding symptoms. The stress of being confined could have triggered it, no way of knowing. Birds cannibalizing a dead flock mate is not uncommon, so the pecking could have happened after she died.

 

[Ebony Rose]

She MIGHT have had a stroke, heart attack, or other undiagnosed ailment that proved fatal and in her death throes brought about the wounds you saw postmortem.
It is VERY common for chickens to 'attack' another bird that's flapping and flopping around.
Just saying, they might be totally innocent of killing her.